ACTIVITY at North Korea's secretive Yongbyon nuclear centre has been ongoing, an analysis has indicated - despite Kim Jong-Un's "unprecedented offer" to Donald Trump to shut it down.

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Yongbyon is at the centre of the hermit state's efforts to construct nuclear weapons, producing the fissile material for the six tests so far ordered by leader Kim Jong-un. Satellite imagery published by the 38 North website suggested railcars which arrived in the area by February 10 had departed by February 26. The analysis, jointly authored written by experts Peter Makowsky, Olli Heinonen, Frank V Pabian and Jack Liu, said: "The movement of the railcars throughout the complex suggests they could have been transporting reagents used in various nuclear fuel production and/or waste handling and decontamination processes.

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"Without corresponding indicators of activity at the Radiochemical Laboratory, they are more likely associated with activity at the suspected Radioisotope Production Plant or Uranium Enrichment Plant (UEP).

"There are no signs that the products transported were radioactive.

"While there are no indications that the 5 MWe Reactor or the Experimental Light Water Reactor (ELWR) are in operation, fresh snowfall present on February 19 revealed continued low levels of activity throughout the complex."

The report adds: "While the exact purpose of these railcars is unclear, the casks or canisters on the railcars could be transporting reagents used in various possible nuclear fuel production processes and/or waste handling and decontamination or to maintain the associated equipment.

Overview of Radioisotope Production Plant and Uranium Enrichment Plant area (Image: Maxar/38 North)

"It is also unknown whether these specialized cars had deposited their cargo while on site or had been loading new cargo before their departure.

"These rail cars have been observed at this spur on a periodic basis since at least 2002.

Clearly it was impossible to know precisely what was happening at Yongbyon, the experts said - adding: "While no unusual activity was observed in the Radiochemical Laboratory area, two probable trucks were seen on the road entering the Fuel Reception Area on February 23.

"Several trucks were also observed in the motor pool area during this period of coverage.

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"Combined with the snow removal from the internal roadways and tire tracks and pathways created in the snow, these factors indicate activity at this facility."

In a rare press conference held in Hanoi late on Thursday, North Korean foreign ministry officials said they had made a “historically unprecedented offer” to close all of Yongbyon together with US experts.

US negotiators confirmed North Korea had made the offer, but said talks broke down over exactly which facilities were included, and the scope of sanctions relief that Pyongyang demanded in return.

Built in the late 1950s with Soviet aid, the Yongbyon complex houses at least three reactors which North Korea says are intended to produce electricity.

It also has a fuel fabrication facility and a plutonium reprocessing plant, where weapons-grade materials can be extracted from spent fuel rods, according to the Nuclear Threat Initiative (NTI), a Washington-based think-tank.

A five-megawatt reactor there produces weapons-grade plutonium, a major source of fuel for the nuclear program.

North Korea nuclear timeline (Image: Daily Express)

Siegfried Hecker, a nuclear scientist who previously headed the Los Alamos weapons laboratory in the United States, said North Korea told him they had 2,000 centrifuges, to produce weapons-grade uranium to build bombs, in operation during his visit to Yongbyon in 2010.

Kim Dong-yub, a military expert at Kyungnam University’s Institute of Far Eastern Studies in Seoul, said there could be up to 6,000 centrifuges in North Korea, and 4,000 of them in Yongbyon, based on studies by Hecker and other agencies.

Toward the end of last year, Kim Jong-un hinted that he was ready to resume missile testing after apparently becoming frustrated with the slowness of negotiations with the United States.